This invention relates to a method of controlling the treatment of wastewater from the manufacture of peroxide bleached mechanical or chemi-mechanical pulp, which treatment is carried out in at least two anaerobic steps and possibly a subsequent aerobic step. The anaerobic steps are at least one hydrolysis step and a subsequent methane fermentation step.
Wastewater from the manufacture of mechanical pulps contains a relatively high proportion of carbohydrates, which advantageously can be attenuated in an anaerobic step by methane bacteria to methane. In the manufacture of peroxide bleached mechanical or chemi-mechanical pulp, however, the wastewater contains a certain content of hydroperoxide. The methane fermentation step, which contains absolute anaerobic bacteria, does not stand hydroperoxide at all, but the bacteria are killed immediately even at low hydroperoxide concentrations. The methane step, for working satisfactorily, requires that the water ingoing to the methane step do not contain measurable contents of hydroperoxide.
The hydroperoxide content in wastewater from peroxide bleaching of the pulp type in question generally amounts to 0.1-0.3 g/l.